The structure of the bovine proteolytic enzyme lambda-chymotrypsin has been further refined at 1.9A resolution by the method of restrained least squares as developed by Hendrickson and Konnert. The acid protease from Rhizopus chinensis has been refined by a combination of restrained least squares and difference electron density maps to 2.0A resolution. Adjustments have been made in the amino acid sequence by the protein based on the features apparent in the electron density map. A preliminary analysis has been made of the mode of binding of the universal acid protease inhibitor, pepstatin.